Wireless cellular communication networks incorporate a number of mobile user equipments (UEs) and a number of NodeBs. A NodeB is generally a fixed station, and may also be called a base transceiver system (BTS), an access point (AP), a base station (BS), or some other equivalent terminology. As improvements of networks are made, the NodeB functionality evolves, so a NodeB is sometimes also referred to as an evolved NodeB (eNB). In general, NodeB hardware, when deployed, is fixed and stationary, while the UE hardware is portable.
In contrast to NodeB, the mobile UE can comprise portable hardware. User equipment (UE), also commonly referred to as a terminal or a mobile station, may be fixed or mobile device and may be a wireless device, a cellular phone, a personal digital assistant (PDA), a wireless modem card, and so on. Uplink communication (UL) refers to a communication from the mobile UE to the NodeB, whereas downlink (DL) refers to communication from the NodeB to the mobile UE. Each NodeB contains radio frequency transmitter(s) and the receiver(s) used to communicate directly with the mobiles, which move freely around it. Similarly, each mobile UE contains radio frequency transmitter(s) and the receiver(s) used to communicate directly with the NodeB. In cellular networks, the mobiles cannot communicate directly with each other but have to communicate with the NodeB.
Control information bits are transmitted, for example, in the uplink (UL), for several purposes. For instance, Downlink Hybrid Automatic Repeat ReQuest (HARQ) requires at least one bit of ACK/NACK transmitted in the uplink, indicating successful or failed circular redundancy check(s) (CRC). Moreover, a one bit scheduling request indicator (SRI) is transmitted in uplink, when UE has new data arrival for transmission in uplink. Furthermore, an indicator of downlink channel quality (CQI) needs to be transmitted in the uplink to support mobile UE scheduling in the downlink. While CQI may be transmitted based on a periodic or triggered mechanism, the ACK/NACK needs to be transmitted in a timely manner to support the HARQ operation. Note that ACK/NACK is sometimes denoted as ACKNAK or just simply ACK, or any other equivalent term. Here, ACK refers to acknowledgement (successful CRC check) and NACK refers to negative-acknowledgement (failed CRC check). The ACK/NACK information is typically required to be highly reliable in order to support an appropriate and accurate HARQ operation. This uplink control information is typically transmitted using the physical uplink control channel (PUCCH), as defined by the 3GPP working groups (WG), for evolved universal terrestrial radio access (EUTRA). The EUTRA is sometimes also referred to as 3GPP long-term evolution (3GPP LTE). The structure of the PUCCH is designed to provide sufficiently high transmission reliability.
In addition to PUCCH, the EUTRA standard also defines a physical uplink shared channel (PUSCH), intended for transmission of uplink user data. The Physical Uplink Shared Channel (PUSCH) can be dynamically scheduled. This means that time-frequency resources of PUSCH are re-allocated every sub-frame. This (re)allocation is communicated to the mobile UE using the Physical Downlink Control Channel (PDCCH). Alternatively, resources of the PUSCH can be allocated semi-statically, via the mechanism of semi-persistent scheduling. Thus, any given time-frequency PUSCH resource can possibly be used by any mobile UE, depending on the scheduler allocation. Physical Uplink Control Channel (PUCCH) is different than the PUSCH, and the PUCCH is used for transmission of uplink control information (UCI). Frequency resources which are allocated for PUCCH are found at the two extreme edges of the uplink frequency spectrum. In contrast, frequency resources which are used for PUSCH are in between. Since PUSCH is designed for transmission of user data, re-transmissions are possible, and PUSCH is expected to be generally scheduled with less stand-alone sub-frame reliability than PUCCH. The general operations of the physical channels are described in the EUTRA specifications, for example: “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation” (3GPP TS 36.211, Release 8).
A reference signal (RS) is a pre-defined signal, pre-known to both transmitter and receiver. The RS can generally be thought of as deterministic from the perspective of both transmitter and receiver. The RS is typically transmitted in order for the receiver to estimate the signal propagation medium. This process is also known as “channel estimation.” Thus, an RS can be transmitted to facilitate channel estimation. Upon deriving channel estimates, these estimates are used for demodulation of transmitted information. This type of RS is sometimes referred to as De-Modulation RS or DM RS. Note that RS can also be transmitted for other purposes, such as channel sounding (SRS), synchronization, or any other purpose. Also note that Reference Signal (RS) can be sometimes called the pilot signal, or the training signal, or any other equivalent term.
In the EUTRA time division duplex (TDD) frame structure, different subframes are allocated for downlink (DL) and uplink (UL) transmissions. One property of TDD is that the number of UL and DL subframes within a radio frame can be different. In configurations where there are more DL subframes than UL subframes, multiple DL subframes are associated with one single UL subframe for the transmission of the corresponding control signals. For example, for each dynamically scheduled transmission in the DL subframes, ACK/NAK bits need to be transmitted in an associated UL subframe to support proper hybrid ARQ (HARQ) operation. If a UE is scheduled in multiple DL subframes which are all associated with one single UL subframe, then the UE needs to transmit multiple ACK/NAK bits in that UL subframe. One solution to reduce the number of ACK/NAK bits the UE needs to transmit is to adopt ACK/NAK bundling. Typically, the UE compresses the multiple ACK/NAK bits into one or two bundled ACK/NAK bits, which are transmitted in the corresponding UL subframe. A common ACK/NAK compression or bundling rule is that if all ACK/NAK bits are of value (ACK), then one (ACK) bit is transmitted in the UL subframe to acknowledge all received packets in the multiple DL subframes. If there is at least one NAK bit, then one (NAK) bit is transmitted in the UL subframe to request retransmission of all DL packets. The general operations of the TDD channels are described in the EUTRA specifications, for example: “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures” (3GPP TS 36.213, Release 8).